1. Field of the Invention
This invention relates generally to chemical and biochemical analysis of an analyte in a fluid or gaseous mixture, and more particularly concerns a composition for a solution for storage and calibration of an intravascular or intra-arterial multiparameter blood gas sensor incorporating at least a pH sensing element and a carbon dioxide sensing element.
2. Description of Related Art
Measurement of acidity (pH) and the tension or partial pressure of carbon dioxide and oxygen in the blood is important in modern medicine for determination of the respiratory condition of a patient. Multiparameter sensors for taking intravascular measurements of acidity, carbon dioxide and oxygen levels in the blood have been developed in which a fluorescent indicator dye is encapsulated within a membrane cover such as a hydrophobic or hydrophilic matrix which is permeable to the analyte to be measured, coupled to one or more optical fibers for measuring the intensity of fluorescence of the indicator dye. The fluorescence intensity of appropriately chosen indicators is altered according to the level of acidity, carbon dioxide, or oxygen being measured, allowing measurement of blood levels of these parameters through a compatible intravascular catheter system.
Aqueous solutions of bicarbonate are commonly useful for calibration of pH sensors, or multiparameter sensors including pH, O.sub.2 and CO.sub.2 sensing elements by gas tonometry, by infusion of carbon dioxide gas into a calibration solution in the tonometer. One prior art reference, for example, discloses isotonic saline solutions containing 1.52 grams per liter of sodium bicarbonate, equilibrated with 10.02% carbon dioxide in nitrogen, or containing 1.9108 grams per liter of sodium bicarbonate equilibrated with 4.80% carbon dioxide, 11.62% oxygen, and the balance nitrogen, for calibration of a pH sensor. Another reference discloses a buffer solution containing 0.01 molar sodium bicarbonate and 1.0 molar sodium chloride, for calibration of a carbon dioxide sensor. A 15 mM bicarbonate buffer solution for calibration of an oxygen and carbon dioxide combination sensor is also known.
Solutions of two or more buffer components are also known. However, such conventional multicomponent buffer solutions typically are designed to cover a broad pH range, and work additively to maintain a desired pH range. These solutions are generally not useful for calibration by gas tonometry.
Calibration of multiparameter pH, CO.sub.2 and O.sub.2 blood gas sensors typically involves tonometry with two different gas compositions containing different gas levels in a calibration solution. The concentrations of both carbon dioxide and oxygen are typically selected to be changed in the gas compositions to bracket the physiological normal levels of these gases in the blood. It would also theoretically be desirable to formulate the calibration solution so that the two different carbon dioxide levels in the two selected gas compositions would result in two different pH levels in the normal physiological range bracketing the normal physiological pH of 7.4. However, this has been found in practice to necessitate formulation of a buffer solution with an initial pH at ambient atmospheric carbon dioxide levels much higher than the physiological normal value, usually greater than a pH of 8.5. Storage of an intravascular multiple parameter sensor in a solution with such a high pH can have deleterious effects on the surface of the multiple sensor which comes in contact with blood, resulting in reduced blood compatibility, especially when a silicone polymer forms a principal part of the surface of the intravascular sensor.
It has also been found that in the process of steam sterilization of intravascular blood gas and pH sensors stored in a buffered calibration solution, the calibration solution can become highly alkaline, often reaching a pH of 9 or more. Under such conditions of high temperature and pH, the surface of the intravascular device which comes into contact with blood can also be chemically altered, and such chemical alterations can also lead to blood compatibility problems. It has also been found that when multiparameter pH, CO.sub.2 and C.sub.2 sensors are stored in such a solution which has been exposed to steam sterilization, such high pH levels can further cause the carbon dioxide sensing element to experience drift of measurements of carbon dioxide. In view of these problems, it would be desirable to provide a solution for storage and calibration of such intravascular blood sensors that is buffered to maintain a pH substantially at or slightly below a near neutral pH, and preferably less than pH 8, in the absence of CO.sub.2 gas in the solution after exposure to the elevated temperatures of steam especially during sterilization.
For calibration of a multiparameter blood gas and pH sensor incorporating a carbon dioxide sensing element, it would be desirable to provide an improved calibration solution in which the sensor can also be stored to maintain a substantially neutral pH without interference with or loss of the gas tonometric properties of the solution. The present invention meets these needs.